We have recently developed a high yield procedure for the large-scale purification of the key alternative complement component, factor B. Subsequently, we determined the amino acid sequence of over 35% of this rather formidable protein (Mr 93,000) including areas of critical functional importance, such as the apparent active site serine and histidine regions. Currently I am involved in attempts to sequence the remaining active site region and the factor D cleavage region of factor B as well as factor D itself. In the hopes of eventually being able to relate host-defense mechanisms with specific diseases, therefore, I would now like to try to correlate the emerging physical structure of factors B, D, and C3b (CVF) with their essential biological functions in an attempt to better understand their interactions during C3 convertase formation. Specifically I plan to: (1) study the substrate specificity of the two enzymatically active components, factors B and D, by examining the cleavage of peptides of known primary amino acid sequence which mimic that of their native substrates, (2) use chemical cross-linking reagents to analyze conformational changes that might be induced in factor B during its association with C3b (CVF) and subsequent cleavage by factor D as well as the interactions between factor B and C3b (CVF) during this activation process, and (3) attempt the cleavage of factor B by factor D in the absence of C3b under various mild denaturing conditions in an effort to resolve whether the cleavage region of factor B or the active site of factor B determines the specificity of this hydrolysis. It is hoped that this proposed research might ultimately lead to an unraveling of the complex molecular interactions regulating the alternative complement pathway and generate further insights into the role and mode of action of this enzyme system in human diseases such as cancer, rheumatoid arthritis, and periodontal disease.